An electrophotographic process is well utilized in a copying apparatus or a printer. In the process, for example, an electrostatic latent image is formed first on a photosensitive medium such as a photosensitive drum and then the electrostatic latent image is visualized by the development with a toner. The visualized image, namely a toner image, can be converted to a copied or printed information by fixing the toner image directly onto the photosensitive medium. However, for the repeated use of the photosensitive medium, it is necessary to transfer the toner image to other transfer medium such as paper. Therefore, the process usually includes a transfer process of the toner image. The transfer utilizes so-called Coulomb force by which polar charge reverse to the toner image, which was given to the transfer medium by corona discharge, pulls the toner of the toner image toward the transfer medium. The toner pulled on the transfer medium is then fixed there by heat or pressure, or both.
Dry toner is a developer used in the above-mentioned development process. It is classified into one-component type and two-component type. The one-component type toner consists of a mixture of resin powder as main part and flow agent which gives a flowability to the resin powder. On the other hand, two-component type toner consists of a mixture of resin powder as main part and magnetizable powder, namely carrier.
The toner development by one-component type toner is carried out by utilizing its own electrostatic induction ability. That is, since the one-component type toner has some electric conductivity, the contrary polar charge to the electrostatic latent image is induced by electrostatic induction when it is brought near the electrostatic latent image. Accordingly, Coulomb force acts between the electrostatic latent image and the toner and the toner is pulled toward the electrostatic latent image.
Thus, in the development process, the one-component type toner must be conductive. Contrariwise, since the transfer utilizes corona discharge as mentioned above, the toner must also be insulative. Namely, the one-component type toner should have a delicate balance of properties: electric conductivity and insulation ability which conflict with each other. In order to always obtain an image of constant density, it is necessary that the balance is stable. However, the property of electric conductivity or insulation ability is nothing else but the problem of the degree of resistivity of the toner. Since the value depends on not only its composition but also largely on the environmental condition, especially humidity of air, it is essentially unstable.
In conventional one-component type toner, silica is used, for example, as flow agent. Since silica particles are very fine and have a large specific surface area in order to increase flowability of the toner, they very readily absorb water. Therefore, it is very difficult to maintain the resistivity of the toner constant and a decrease in flowability of the toner due to absorbed humidity also occurs.
On the other hand, another one-component type toner is known, wherein hydrophobic silica is used as flow agent. All active sites of this type of silica, where water is absorbed, namely hydroxyl (OH) radical at the surface, are chemically blocked with dimethyldichlorosilane, etc. Consequently, the hydrophobic silica has little tendency to absorb water and thus the resistivity of the toner is kept almost constant irrespective of the level of humidity in air. However, since, in this toner, there are no polar OH radicals at the surface of hydrophobic silica flow agent, the flow agent has become neutral and can have no definite polar charge. The resin powder alone gives insufficient polarity to the toner and an admixture with charge control agent is recommended. Then, the charge control agent itself will be humidified, making it difficult again to maintain constant resistivity of the toner and decreasing the flowability of the toner.
In contrast with such one-component type, in the two-component type toner which will be explained hereinbelow, the only requirement is insulation ability. Namely, toner development with the two-component type toner is carried out by pulling the toner to the electrostatic latent image with an electrostatic force generated through frictional charge between resin powder and carrier. Thus, the transfer process utilizes the insulation ability of the toner and there is no need for delicate control, as is necessary with one-component type toner. However, in the two-component type toner, resin powder is consumed while carrier remains unused for repeated use. So it is necessary to make up the former to keep the concentration of the same and the carrier itself also becomes ineffective through repetitious use. With the one-component type toner, such troublesome maintenance as concentration adjustment and replacement of carrier is totally unnecessary.